In the conception and design of optical components it is necessary to take account of the interaction of the materials used with the nature of the irradiated light. In one derived version the law of conservation of energy takes the formT(λ)+p(λ)+a(λ)=1,where T(λ) describes the fraction of light transmitted, p(λ) the fraction of light reflected, and a(λ) the fraction of light absorbed (λ: wavelength), and where the total intensity of the irradiated light has been normalized to 1. Depending on the application to which the optical component is put the task is to optimize individual terms among these three and to suppress the others in each case. Optical components designed for transmission ought to feature values for T(λ) of close to 1. This is achieved by reducing the value of p(λ) and a(λ). PSAs based on acrylate copolymer and acrylate block copolymer normally have no significant absorption in the visible range, i.e., in the wavelength range between 400 nm and 700 nm. This can be readily ascertained by measurements with a UV-Vis spectrophotometer. The factor of particular interest, therefore, is p(λ). Reflection is an interfacial phenomenon which depends on the refractive indices nd,i of two phases i in contact, according to the Fresnel equation
      ρ    ⁡          (      λ      )        =                    (                                            n                              d                ,                2                                      -                          n                              d                ,                1                                                                        n                              d                ,                2                                      +                          n                              d                ,                1                                                    )            2        .  
For the case of isorefractive materials, for which nd,2=nd,1, p(λ)=0. This explains the need to adapt the refractive index of a PSA that is to be used for optical components to those of the materials to be bonded. Typical values for a variety of such materials are set out in Table 1.
TABLE 1MaterialRefractive index ndQuartz glass1.458Borkron (BK7)1.514Borkron1.518Flint1.620(Source: Pedrotti, Pedrotti, Bausch, Schmidt, Optik, 1996, Prentice-Hall, Munich. Data for X = 588 nm)
One concrete application to the bonding of an optical component is the bonding of optically active films for liquid-crystal-based display modules (known as LC displays), which significantly enlarge the angle of vision of the viewer. The attachment of such films is subject to stringent requirements. For instance, the adhesive ought to be highly transparent, so that the luminance of the LC display is only slightly reduced. This can be achieved, in accordance with the exposition above, by minimizing the fractions of absorbed and reflected light. It is therefore necessary to adapt the refractive index of the adhesive to that of the optical component to be joined.
Besides the adhesive bonding of optically active films to LC displays, however, there also exist a multiplicity of other applications, including those, for example, for the eye sector, which impose additional requirements on the adhesive.
Generally speaking it is possible, for the purpose of adhesive bonding, to use liquid adhesives (obtainable, for example, from Norland), which cure to form a solid bond. The disadvantage of liquid adhesives, however, is that, first, the glass or the optical film to be bonded comes into contact with solvents, something which in certain cases ought to be avoided. Additionally, an inherent disadvantage of liquid adhesives as compared with pressure-sensitive adhesives is that they require a lot of time for the drying and curing process.
As a result of their inherent tackiness, PSAs must possess a relatively low glass transition temperature. This limits the aromatics fraction and hence the maximum refractive index. There nevertheless exists a variety of types of PSA which attain very high refractive indices. Silicone rubbers are extremely suitable here (U.S. Pat. No. 4,874,671), but are frequently ruled out of use on cost grounds.
In U.S. Pat. No. 5,639,530, furthermore, styrene-dienes, block copolymers, are used for bonding reflective materials. U.S. Pat. No. 6,266,166 uses a pressure-sensitive acrylate adhesive tape to produce holograms. There, by addition of a highly transparent resin equipped with a high refractive index, the refractive index is raised to about 1.52. A disadvantage again, however, is the addition of resin, which significantly impairs the fogging behavior of the acrylate PSA. The same is true of the other additions too, such as aromatics or halogen compounds, for example.
It is an object of the invention to provide high-transparency pressure-sensitive adhesives having good adhesive properties on the basis of a large selection of acrylate-based monomers, the pressure-sensitive adhesives featuring, in particular, low fogging and/or outgassing behavior.